1. Field of the Invention
The present invention relates to system multiplexing for forming one stream of digitally compressed image and audio data and, more specifically, to a system multiplexing apparatus preventing an overflow of a buffer in an audio decoder.
2. Description of the Background Art
Recently, along with performance improvement of semiconductor devices, methods of digitally compressing images and sounds have been vigorously studied, and international standard specifications for digital compression of images and sounds, including MPEG (Moving Picture Experts Group) specification, have been determined. In compliance therewith, video recording and reproducing apparatuses for digital TV broadcast, DVD (Digital Versatile Disc) and the like utilizing digital compression are becoming popular.
When moving images and sounds are to be handled simultaneously, a process referred to as system multiplexing is necessary, by which digitally compressed image and audio data are turned into one stream. In the system multiplexing process, image data and audio data are divided into small units, and the data are arranged in order considering amounts of data and time points of display and reproduction, to form one collective stream.
Japanese Patent Laying-Open No. 8-98160 discloses a related technique. A data multiplexing apparatus disclosed in Japanese Patent Laying-Open No. 8-98160 includes a plurality of encoders encoding input digital sounds and moving images and outputting the result as an encoded stream, multiplexes a plurality of such encoded streams to enable simultaneous reproduction, and the apparatus includes control means for performing virtual decoding buffer simulation for each of the encoded streams and outputting a multiplex request signal based on an amount of occupation by the data in the buffer, and multiplexing means for multiplexing each of the encoded streams based on the multiplex request signal.
In the data multiplexing apparatus disclosed in Japanese Patent Laying-Open No. 8-98160 mentioned above, virtual decoding buffer simulation is performed for each encoded stream and multiplexing is done based on the amount of data occupation in the buffer. When audio completing of a VOBU (video object unit) boundary is performed, however, an overflow may occur undesirably in a buffer of an audio decoder, as will be described later.